Yes, I think you are close on this. To confirm, I want to read in the values specific to the form that is being parsed through PHP - not just the lead fields. So the process would be:
1. URL to the form is passed to PHP
2. Parse the form, confirms what fields exist and whether they are required or not
3. Confirm which fields have already been filled out for the lead based on cookie reference
4. Show remaining # of progressive fields based on Marketo form settings (show the next 3 fields for instance) and on what is not already recorded in the lead
Is this possible?